1. Field of the Invention
The invention relates to a nonaqueous electrolyte secondary battery and a battery module.
2. Description of the Related Art
A nonaqueous electrolyte secondary battery using a lithium metal, lithium alloy, lithium compound or carbon material for its negative electrode is expected as a high energy density battery or high output density battery, and it has been intensively researched and developed. So far, lithium batteries comprising a positive electrode containing LiCoO2 or LiMn2O4 as an active material and a negative electrode containing a carbon material for intercalating and deintercalating lithium ions have been widely put in practical use. In the negative electrode, various materials which are a substitute for the carbon material are being studied, such as metal oxides and alloys.
In a nonaqueous electrolyte secondary battery, a copper foil is generally used as a current collector of a negative electrode. A lead and a terminal to which the lead is connected are usually made of copper or nickel. A secondary battery comprising a negative electrode including a current collector of copper foil is elevated in potential of the negative electrode in an overdischarged state. Accordingly, the dissolving reaction of the negative electrode made of copper foil is promoted, and the discharge capacity drops suddenly. When a long cycle continues in a battery module, a battery capacity balance is broken, and overdischarge may occur in certain batteries. For this reason, the current collector made of copper foil, which is assembled in a battery in an overdischarged state begins to dissolve. Therefore, the secondary battery is provided with a protective circuit for preventing itself from being overdischarged.
However, since such a protective circuit is installed, the secondary battery is reduced in energy density. If an outer container having a thin metal can is used in order to reduce the battery weight, copper for composing the current collector of the negative electrode, lead and terminal is dissolved, for example, at the time of overdischarging, and swelling of the battery increases.
Hence, JP-A. 2004-296256(KOKAI) discloses a nonaqueous electrolyte secondary battery using an aluminum foil or aluminum alloy foil for a negative electrode current collector, in use of a negative electrode active material for intercalating lithium ions at a specific potential. Such a configuration makes it possible to realize a nonaqueous electrolyte secondary battery enhanced in energy density and overdischarge cycle performance. Further, since this nonaqueous electrolyte secondary battery can elevate the discharge capacity over several Ah or tens Ah, it is highly expected to be used as a square nonaqueous electrolyte secondary battery for use in, aside from electric power storage, vehicles such as a power-assisted bicycle, electric scooter, electric vehicle, hybrid vehicle, and electric train.
An on-board secondary battery is required to be low in internal resistance, high in energy density, and high in output density, for the purpose of obtaining high output. Further, excellent cycle performance, and high strength and corrosion resistance of materials for a long period are demanded in the conditions of high temperature, high humidity, vibration, quick charging, high output discharge, and overdischarge. Therefore, for the purpose of maintaining the internal resistance of the battery at a low level, a connecting portion between a lead and a terminal where current in the battery is concentrated is required to be high in mechanical strength, electrochemical stability, and chemical stability in the high temperature and high humidity environment for a long period, thereby maintaining low resistance excellent in corrosion resistance.
However, when in the secondary battery described above, the lead and terminal of the negative electrode are formed of copper with an excellent conductivity, a corrosion or dissolving reaction may be advanced in the connecting portion between the lead and terminal in an overdischarge operation of the battery or high temperature and high humidity environment over a long period of use, and thereby the resistance is increased. For this reason, it becomes difficult to obtain high output from the battery and battery module. Further, since the impedance is increased, the discharge reaction of the positive electrode and negative electrode is not promoted sufficiently in high output discharge, and a utility rate of an active material is lowered.
On the other hand, JP-A 2003-36825(KOKAI) discloses a nonaqueous electrolyte battery high in safety, the battery having a terminal structure capable of preventing or suppressing sparks possibly occurring in a terminal unit due to leak of a nonaqueous electrolyte. This document, in paragraph [0036], discloses that a negative electrode terminal 40 made of copper alloy is composed of a current collector 40a serving as an internal terminal and a bolt-like external terminal unit 40b; that the current collector 40a is bonded to a band-like negative electrode lead 12a extending from an electrode body 10; that the external terminal unit 40b is positioned outside of a negative electrode side lid plate 23 of a battery case 20; and that the exposed surface portion is plated with aluminum to prevent spark generation.
In the nonaqueous electrolyte battery, the current collector 40a of the negative electrode terminal 40 to which the negative electrode lead is connected is made of copper alloy. Therefore, as mentioned above, a corrosion or dissolving reaction is promoted in a connecting portion between the lead and terminal in an overdischarge operation of the battery or high temperature and high humidity environment over a long period of use, and the internal resistance increases. Consequently, it becomes difficult to obtain a high output from the battery or battery module.